1. Field of the Invention
The present invention relates generally to solid-state imaging devices, and more specifically to a solid-state imaging device of active-pixel type where each pixel includes a transistor for amplifying the voltage developed by an associated photodiode.
2. Description of the Related Art
A prior art solid-state imaging device of the active-pixel type, shown in FIG. 1, includes a plurality of active pixels 100 arranged in a matrix pattern of rows and columns, with the rows having associated row select lines 105 and row reset lines 106 connected to a row scanner 110 and the columns having associated column select lines 108 connected to a column scanner 111 via associated column select circuits 120. Each active pixel consists of a photodiode 101, a row reset transistor 102, an amplifying transistor 103 and a row-select transistor 104. When a given row is accessed by the row scanner, the associated reset line 106 is driven high and all of its row-reset transistors 102 are turned on, allowing voltages to be injected into all of its photodiodes 101 from an associated power (source voltage) line 107. The reset line 106 of the given row is then driven low to turn off all of its reset transistors 102. This allows the photodiodes of the given row to be exposed to incident light for a predetermined amount of time. During this exposure, currents are drained to ground from the photodiodes in proportion to the intensities of light incident on the respective photodiodes. At the end of the exposure time, the row select line 105 of the given row is driven high so that its row select transistors 104 are turned on. All the outputs of its photodiodes 101 are amplified by the corresponding transistors 103 and the amplified voltages appear at the column select lines 108, respectively.
Each column-select circuit 120 includes a constant-current source transistor 109 (which is biased by a suitable voltage on a bias line 115) and a column-select transistor 112. For each column, the constant-current source transistor 109 forms a source follower with the amplifying transistor 103 of a row selected pixel 100 of the column. While a given row is selected, the column select transistors 112 are scanned by the column scanner 111 so that they are sequentially turned on to transfer the voltages respectively developed at the column-select lines 108 to an output line 113, where the signals are serially read out into external circuitry via an output buffer 114.
If the source-followers are implemented so that their amplification gain meets the design objective, the voltage developed at each column-select line 108 is equal to the voltage at the gate of each amplifying transistor 103 (i.e., the voltage developed by each photodiode 101). However, due to the effect of a backplane bias of the integrated-circuit chip, the voltage developed at each column select line is not equal to the voltage developed by each photodiode. Therefore, the actual gain of the source follower falls far short of the ideal, typically at 75 percent of the design objective. If the photodiodes can deliver an output voltage of from zero to 3 volts, the voltage at the column select lines will only swing in the full-scale range between zero and 0.23 volts. The prior art solid-state imaging device of this type is therefore less robust to external noise. Moreover, since the reset voltage of the photodiodes is supplied from the power lines 107, the full-scale range of the photodiodes is determined by the power supply voltage. Further, the recent trend of the integrated circuit technology is toward using low power supply voltages. If the imaging device were to operate on a lower voltage, it is obvious that it will suffer from the inability of the photodiodes to deliver a sufficient voltage. Therefore, a costly, high precision analog-to-digital converter would be needed to resolve the small amplitude signals.
It is therefore an object of the present invention to provide a solid-state imaging device capable of delivering high amplitude output signals.
In broader terms, the solid-state imaging device of the present invention comprises a plurality of photodiodes, a plurality of first amplifying transistors for respectively amplifying voltages developed by the photodiodes, a plurality of switching transistors corresponding to the first amplifying transistors for selectively coupling one of the amplified voltages to a common conductor in response to a control signal, and at least one second amplifying transistor connected to the common conductor. The second amplifying transistor jointly forms a voltage follower with each of the first amplifying transistors when each of the switching transistors is turned on in response to the control signal for amplifying the voltage coupled to the common conductor.
In a narrower aspect, the solid-state imaging device of this invention comprises a plurality of photodiodes, a plurality of first amplifying transistors for respectively amplifying voltages developed by the photodiodes, a plurality of switching transistors responsive to a control signal for coupling the amplified voltage to a plurality of conductors; and a plurality of second amplifying transistors respectively connected to the conductors, the second amplifying transistors and the first amplifying transistors jointly forming a plurality of voltage followers when the switching transistors are turned on in response to the control signal for respectively amplifying the voltages coupled to the conductors.
In the disclosed embodiment of the invention, the solid-state imaging device comprises a plurality of row lines, a plurality of column lines, and an array of pixels arranged in a matrix pattern of rows and columns. Each pixel comprises a photodiode for developing a voltage corresponding to light incident thereon, a first amplifying transistor for amplifying the voltage and a row select switching transistor responsive to a row select signal from a corresponding one of the row lines for coupling the amplified voltage to a corresponding one of the column lines. A plurality of second amplifying transistors are respectively connected to the column lines. When the row select switching transistors of the pixels in the selected row are turned on in response to the row select signal, the second amplifying transistors and the first amplifying transistors of the pixels arranged in a selected one of the rows jointly constitute a plurality of voltage followers for respectively amplifying the voltages coupled to the column lines. A plurality of column select switching transistors are provided for sequentially coupling outputs of the voltage followers to an output line in response to column select signals.